Modelform initial values override instance values when both are set

[Alper Cugun]

We ran into a bug (on our side) where our data was not being displayed in a ModelForm for existing instance because we were setting both initial values as the instance in the ModelForm.

Digging into the documentation it turns out that for [ModelForm https://docs.djangoproject.com/en/2.2/topics/forms/modelforms/#providing-initial-values] initial overrides instance values ("Initial values provided this way will override both initial values from the form field and values from an attached model instance.") whereas for normal [Forms ​https://docs.djangoproject.com/en/2.2/ref/forms/api/#dynamic-initial-values] initial does not take precedence over bound data ("These values are only displayed for unbound forms, and they’re not used as fallback values if a particular value isn’t provided."). Maybe that's my mistake for thinking a bound Form and a ModelForm with an instance are roughly equivalent.

I tracked this down to [this commit ​https://github.com/django/django/commit/51dc4ecf943d1dcc044ed956925760f9d480f56c] 12 years ago and it seems that nobody has complained much about this piece of code in the mean time. So maybe it isn't a problem but I think there is something to be said for reversing the precedence in ModelForm.

I've attached a draft diff to fix this.

[Alper Cugun]

draft patch